Thermal storage facility especially suitable for concentrating solar power installations

ABSTRACT

A thermal storage facility stores thermal energy at elevated temperature. The facility includes an enclosure for retaining a thermal storage medium; primary heat transfer surfaces for transferring heat from a circulating heat transfer fluid; and secondary heat transfer surfaces for transferring heat from the thermal storage medium to steam pipes associated with a power generation facility. The thermal storage medium is a metallic phase change material, preferably having a melting temperature above 500° C., interposed between the primary heat transfer surfaces and the secondary heat transfer surfaces. Solar power installations can incorporate one or more of the thermal storage facilities.

FIELD OF THE INVENTION

This invention relates to a thermal storage facility that is especiallysuitable for use in storing thermal energy and especially, although notexclusively, thermal energy that has been derived from the sun using aconcentrating solar power installation.

BACKGROUND TO THE INVENTION

One of the advantages of concentrating solar power (CSP) with a solarreceiver mounted on a tower, is that thermal energy storage is madepossible. Thermal storage is required in order to buffer the powergenerating cycle of sunny, clear days so that a solar power plant isenabled to supply electrical energy at night time and during badweather.

Current thermal storage systems mostly operate in the sensible heatmode, and either store thermal energy in a molten salt of some kind, orstore the heat in solid bodies, like concrete. The use of molten salthas the disadvantages that solidification of the salt may occur in thereceiver loop at night; trace heating is therefore needed to keep saltsin a liquid form during non operational times such as night time and inauxiliary piping; and this may lead to maintenance problems.

Other phase change concepts have been noted but they are all based onlow conductivity salts, and require extensive heat transfer modificationto improve their thermal conductivity.

Solid storage materials, such as concrete bodies, also have limitedpower output and, furthermore, may suffer from thermal stress thatlimits the useful life of such media. Modifications to the solid storageelements makes them expensive.

In other storage systems, oil may be used as a storage medium and heattransfer fluid. One of the primary problems with this is the fact thatthe maximum operating temperature of oil is about 400° C. This seriouslylimits the maximum receiver temperature, and curbs the maximumefficiency of the power plant.

As far as applicant is aware all of the current modes of thermal storagehave limitations. Generally there are two limitations to thermal energystorage, one is a temperature limitation, and the other is high meltingpoint heat transfer fluids that causes a inherent blockage problem.

There is accordingly a need for an alternative thermal energy storagesystem that eliminates the need for trace heating of the heat transferfluid, and offers high power thermal energy storage at highertemperatures than currently practiced.

SUMMARY OF THE INVENTION

In accordance with a first aspect of this invention there is provided athermal storage facility for storing thermal energy at elevatedtemperature, the facility comprising an enclosure for retaining athermal storage medium, primary heat transfer surfaces for transferringheat from a circulating heat transfer fluid, and secondary heat transfersurfaces for transferring heat from the thermal storage medium to steampipes associated with a power generation facility, the thermal storagefacility being characterized in that the thermal storage medium is ametallic phase change material (PCM) that is interposed between theprimary heat transfer surfaces and the secondary heat transfer surfacesand in that the heat transfer fluid is a liquid metal.

Further features of the invention provide for the metallic phase changematerial to have high thermal conductivity, high latent heat of fusionwith an high melting temperature, preferably above about 500° C., andhigh operating temperatures. One candidate material is a metallic phasechange material that is an alloy of aluminium and silicon, preferably acommercially available alloy such as one containing 87.76% Al and 12.24%Si (AlSi12 which is also known as LM6 casting alloy).

The invention further provides for the metallic heat transfer fluid tobe liquid over an extended range of temperatures from ambienttemperature through to a maximum operating temperature in excess of themelting point of the metallic phase change material, with the metallicheat transfer fluid typically being molten alkali metals or alloys ofalkali metals and especially an alloy of sodium and potassium such asthat known as NaK; and for the heat transfer surfaces to be those ofgenerally parallel pipes extending through or adjacent the enclosure.

The invention also provides a thermal storage facility as defined abovein which the thermal storage facility forms part of a unit selected froma pre-heater, a boiler, a super heater, a re-heater and a combinationunit designed to perform the function of any two or more of such units.

In one variation of the invention a solar power installation that istypically of a relatively small capacity, may have a single heliostatfield and associated solar receiver mounted on a tower forming part ofthe solar power installation in which instance the thermal storagefacility as defined above may be built into the tower.

In another variation of the invention a solar power installation maycomprise multiple heliostat fields with associated solar receiverscarried on towers with each of the solar receivers being connected toone or more common thermal storage facilities as defined above. In suchan instance a thermal storage facility may be embodied in any one ormore units selected from a pre-heater, a boiler, a super-heater, and areheater, typically all of these, with the flow of heat transfer fluidto each of the facilities being controlled to achieve the relevantobjective of the relevant unit

It will be understood that with the construction of a thermal storagefacility as defined above, suitable molten alkali metal alloys can beemployed as heat transfer fluid whilst the presence of the metallicphase change material between the molten hot alkali metal alloy andwater or steam prevents contact from being made between the alkali metaland the water or steam, which could otherwise have disastrous results.The arrangement enables the particular properties of materials notheretofore used in the relevant situations to be usefully employed andadvantage to be taken of their special properties. The inventiontherefore enables the water or steam heater or generator to be combinedwith the thermal storage unit.

In use therefore, heat may be transferred from a receiver atop a towerby molten heat transfer fluid, typically in the form of molten alkalimetal alloys such as NaK (eutectic or hypoeutectic), to the metallicphase change material that melts to thereby absorb thermal energy by wayof the material's latent heat of fusion with the heat being stored andtransferred to the secondary heat exchange surfaces to generate steamfrom water and superheat it for use in a power generation plant, as maybe required.

In order that the above and other features of the invention may be morefully understood, two different proposed embodiments thereof will now bediscussed in further detail with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic illustration of one variation of concentratingsolar power installation embodying the facility of the invention whereina single solar receiver is located atop a tower that carries it;

FIG. 2 is a similar illustration of a another variation of concentratingpower installation embodying the facility of the invention in whichmultiple solar receivers are located atop multiple towers that carrythem and heat transfer fluid is fed to multiple thermal storagefacilities that utilise the heat to heat water or steam as may berequired; and,

FIG. 3 is a schematic cross-section taken through one form of a storagevessel according to the invention showing the primary and secondary heattransfer pipes distributed through the metallic phase change material.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the embodiment of the invention illustrated in FIG. 1 theconcentrating solar power installation includes a substantiallyconventional field of heliostat (1) with a solar receiver (2) mounted ona tower (3). The solar receiver on the tower is to be used to heat ametallic heat transfer fluid in the form of the alkali metal NaK.

In this particular embodiment of the invention, the thermal storageunit, steam generator and circulation system of the heat transfer fluid,are all embodied in the tower. It is envisaged that such an arrangementwill only be applicable to certain sizes of concentrating solar powerinstallation and not to larger ones that will most likely be more likethe embodiment of the invention described with reference to FIG. 2.

Reverting to the embodiment of the invention illustrated in FIG. 1, theheat transfer fluid is circulated by means of a pump (4) through thereceiver and through metallic phase change material (5) contained withinthe enclosure (6) substantially filling the tower by way of a series ofheat transfer pipes (7). The heat transfer fluid, namely the molten NaKheat transfer fluid, is thus heated in the thermal receiver andcirculated through the series of heat transfer pipes (7) that define theprimary heat transfer surfaces.

The steam pipes (8) that define the secondary heat transfer surfaces arealso in contact with the metallic phase change material but are spacedapart from the heat transfer pipes (7). The steam pipes extend between alower reservoir (9) and an upper steam chamber (10) or there may be anexternal steam drum.

The remainder of the concentrating solar power installation issubstantially conventional and includes the usual steam treatmentequipment, high pressure, medium pressure, and low pressure turbines(11, 12, 13), and condensation recycling equipment generically indicatedby numeral (14).

It is to be mentioned that in this instance a re-heater (15) forreheating steam leaving the high-pressure turbine (11) and preparatoryto it entering the medium pressure turbine (12), can be constructed in asimilar manner and have its own heliostat field (16), receiver, towerand thermal storage facility.

Turning now to the embodiment of the invention illustrated in FIG. 2, ina larger concentrating solar power installation, a plurality, in thisinstance four, heliostat fields (21) each have their own receiver (22)mounted on a tower (23) with their heat transfer fluid outputs beingconnected together so that the entire solar power receiver assembly cansupply a separate arrangement of thermal storage units that form part ofwater and steam heating and generating units.

In this particular instance the combination of thermal storage and waterand steam heating and generating units include a pre-heater (24), aboiler (25), a super-heater (26) and a re-heater (27) that is interposedbetween the high-pressure turbine (28) and medium pressure turbine (29).In each instance the relevant unit has its own enclosure that iscomposed of heat transfer pipes and steam pipes that have not beenseparately shown as they are arranged in substantially the same way asthose described below with reference to FIG. 3.

As illustrated in FIG. 3, in each instance in which a thermal storagearrangement according to the invention is used, a preferred general typeof arrangement of steam pipes (31) and heat transfer fluid pipes (32) isachieved by distributing the pipes throughout the interior of acontainment enclosure (33) that is filled with metallic phase changematerial (34) in a suitable pattern, such as on a series of concentriccircles.

The desirable properties of the metallic heat transfer fluid include lowmelting point (preferably below room temperature); high thermalconductivity; high density; and high specific heat capacity. A lowmelting point is very important for a reliable concentrating solar powerplant, as the receivers will be cold for at least half of their lifecycle. A high melting point heat transfer fluid in the receivers wouldreduce the reliability of the installation since solidification of theheat transfer fluid may pose significant problems.

NaK is a eutectic mixture of sodium and potassium and its eutecticcomposition is 78% potassium and 22% sodium (by mass). It is a liquidbetween −12.6° C. and 785° C. Compositions between 40% and 90% potassium(by mass) are liquid at room temperatures. Mixtures with more sodium arepreferred, since the specific heat capacity of sodium is more than thatof potassium. The properties of NaK include high reactivity with water;if it is stored in air, it forms a potassium super oxide that can igniteand is highly reactive with organics which makes it dangerous to storein organic solvents and mineral oil. Eutectic NaK has a very highsurface tension; a density of 0.855 g/mL at 100° C.; and a thermalconductivity of 23.2 W.m⁻¹.K⁻¹ @100° C.

Years of research in the nuclear industry established a standard NaKdesign and handling protocol so that NaK can be safely used in aconcentrating solar power plant.

Regarding the metallic phase change material, two good contenders are analloy composed of 87.76% Al and 12.24% Si that has a melting temperatureof 557° C. and a heat of fusion of 498 J/g; and an alloy composed of83.14% Al, 11.7%Si and 5.16% Mg that has a melting temperature of 555°C. and a heat of fusion of 485 J/g. The former was chosen because it isa common and low cost casting alloy with high thermal conductivity. Itis non toxic, readily available, and there are prospects of improvingits latent heat of fusion through further alloying.

The upper receiver temperature of over 770° C., and the meltingtemperature of the AlSi alloy at 557° C. makes the use of a steam cyclepossible. Superheated steam can therefore be produced directly from heatstored in the metallic phase change material.

There are a number of ways the heat transfer concept can be implemented,but the final geometry of the concept will be dictated by the heattransfer requirements of the specific design. The containment enclosuredoes not need to be cylindrical, but in the instance of a tower, acylindrical form makes sense.

A regards possible pipe layouts associated with the containmentenclosure, more specific geometries may be suitable for particularapplications. One factor that needs to be considered is the heat to betransferred by way of the primary heat transfer surfaces being those ofthe heat transfer fluid pipes. Another factor is the distance betweenthose heat transfer fluid pipes and the secondary heat transfer surfacesbeing those of the steam pipes in order to provide for the required heatflux. Another factor is the volume of the metallic phase change heatstorage material present. Naturally the heat transfer requirements aresize dependant.

It will be understood that numerous possibilities exist and that theembodiments of the invention described above are simply illustrative ofpossible arrangements.

The invention therefore provides an effective thermal storage facilitythat is especially suitable for use in conjunction with heliostat fieldreceiver plants.

1. A thermal storage facility for storing thermal energy at an elevatedtemperature, the facility comprising an enclosure configured to retain athermal storage medium, primary heat transfer surfaces configured totransfer heat from a circulating heat transfer fluid, and secondary heattransfer surfaces configured to transfer heat from the thermal storagemedium to steam pipes associated with a power generation facility,wherein the thermal storage medium is a metallic phase change materialthat is interposed between the primary heat transfer surfaces and thesecondary heat transfer surfaces and wherein the heat transfer fluid isa liquid metal.
 2. A thermal storage facility as claimed in claim 1,wherein the metallic phase change material has high thermalconductivity, a high latent heat of fusion with and a high meltingtemperature.
 3. A thermal storage facility as claimed in claim 2,wherein the melting temperature of the metallic phase change material isabove 500° C.
 4. A thermal storage facility as claimed in claim 3,wherein the metallic phase change material is selected from the groupconsisting of an alloy of aluminium and silicon comprising 87.76% Al and12.24% Si, and an alloy comprising 83.14% Al, 11.7% Si and 5.16% Mg. 5.A thermal storage facility as claimed in claim 1, wherein the metallicheat transfer fluid is liquid over an extended range of temperaturesfrom ambient temperature through to a maximum operating temperature inexcess of the melting point of the metallic phase change material.
 6. Athermal storage facility as claimed in claim 5, wherein the metallicheat transfer fluid is selected from the group consisting of a moltenalkali metal and an alloy of alkali metals.
 7. A thermal storagefacility as claimed in claim 6, wherein the metallic heat transfer fluidis an alloy of sodium and potassium.
 8. A thermal storage facility asclaimed in claim 1, wherein the heat transfer surfaces are surfaces ofgenerally parallel pipes extending through or adjacent the enclosure. 9.A thermal storage facility as claimed in claim 1, wherein the thermalstorage facility forms part of a unit selected from the group consistingof a pre-heater, a boiler, a super-heater, a re-heater and a combinationunit designed to perform the function of any two or more of such units.10. A solar power installation having a single heliostat field andassociated solar receiver mounted on a tower, wherein a thermal storagefacility according to claim 1 is built into the tower.
 11. A solar powerinstallation comprising multiple heliostat fields with associated solarreceivers carried on towers, wherein each of the solar receivers areconnected to one or more common thermal storage facility as defined inclaim 1, wherein the thermal storage facility is embodied in any one ormore units selected from the group consisting of a pre-heater, a boiler,a super-heater, and a re-heater, and wherein the flow of heat transferfluid to each of the facilities is controlled to achieve an objective ofthe one or more units.